Signal repeaters can be used to extend the range of signals by receiving a signal from a source and retransmitting the signal to a mobile device or other receiver. A signal repeater can thus extend the range of a signal by, for example, retransmitting an attenuated signal at a higher power. A signal repeater can also extend the range of a signal by, for example, relaying a signal from one side of an obstacle that may otherwise obstruct the signal to another side of the obstacle, such as the walls of a building or other structure that may block wireless signals.
A signal repeater can be used to provide signal coverage for mobile devices in or associated with moving vehicles, such as planes, trains, automobiles, ferries, etc. A repeater can be installed on or in a vehicle. The repeater can amplify signals communicated between mobile devices in or associated with the vehicle, such as cellular phones, and fixed sources or receivers, such as base stations for telecommunication service providers. The movement of the repeater relative to a source and/or a receiver can cause a frequency shift in the signal due to the Doppler effect. A sufficiently large frequency shift can degrade the ability of a receiver to demodulate, decode, or otherwise process the signal.
For example, a downlink signal can be transmitted at frequency fd from a base station to one or more mobile devices in a train carriage via a repeater installed in the train carriage. The movement of the repeater can cause the repeater to receive the downlink signal from a base station at a frequency fd+fΔ, d and transmit the downlink signal to mobile devices at a frequency fd+fΔ, d, where fΔd is the Doppler shift in a downlink signal caused by the movement of the repeater relative to the base station. Uplink signals can be transmitted from the mobile devices to the base station via the repeater at frequency fu and received by the base station at fu+fΔ, u due to the Doppler shift fΔ, u in the uplink signal caused by the movement of the repeater and relative to the base station. The frequency shift error at the base station can be compounded by mobile devices using the frequency of the signals received from a base station as a reference. For example, a mobile device configured to use frequency fx received from a base station may instead use the frequency fx+fΔ, d as a reference signal. A Doppler shift in the signal transmitted from the mobile device via the repeater at frequency fx+fΔ, d from the mobile device may cause the signal to be received by the base station at frequency fx+fΔ, d+fΔ, u.
Although discussed with reference to a mobile repeater, any communication device having a relative velocity with respect to a signal source or signal destination can experience a frequency shift in signals due to the Doppler effect. Accordingly, systems and methods that can correct the frequency shift in a communications device moving relative to one or more sources or destinations are desirable.